L4- physiology of micturition.pptx
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Transcript L4- physiology of micturition.pptx
Physiology of micturition
Urinary Bladder
Anatomical consideration:
Wall of bladder contain smooth muscle in
different arrangement (detrusor muscle),
contraction emptying of bladder
during micturition.
Muscle bundles (internal urethral
sphincters) in either side of urethra,
made of smooth muscle.
External urethral sphincter, made of
skeletal muscle.
Detrusor – smooth muscle of
the bladder wall
Trigone – smooth
muscle at bladder base
Internal sphincter – smooth
muscle at the bladder neck
External urethral sphincter –
skeletal muscle
Movement of Urine
Wall of ureter contain spiral,
longitudinal and circular bundles of
smooth muscle
Peristaltic contractions occur 1 to 5
times per minute
Moves the urine from pelvis to bladder
Micturition
Once urine enters the renal pelvis, it flows through the ureters
And enters the bladder, where urine is stored.
Micturition is the process of emptying the urinary bladder.
Two processes are involved:
(1) The bladder fills progressively until the tension in its wall is
above a threshold level, and then
(2) A nervous reflex called the micturition reflex occurs that
empties the bladder → at 150-200mls of urine volume
The micturition reflex is an autonomic spinal cord reflex:
however, it can be inhibited or facilitated by centers in the
brainstem and cerebral cortex.
Micturition reflex
stretch
receptors
1) APs generated by stretch receptors
2) reflex arc generates APs that
3) stimulate smooth muscle lining bladder
4) relax internal urethral sphincter (IUS)
5) stretch receptors also send APs to Pons
6) if it is o.k. to urinate
–APs from Pons excite smooth muscle of bladder and
relax IUS
–relax external urethral sphincter
7) if not o.k.
–APs from Pons keep external
urethral sphincter contracted
stretch
receptors
Micturition (Voiding or Urination)
Reflex Control
Urinary bladder distension reaches the conscious
level to void at 150-200 mls.
In adult, volume of urine in bladder that initiates
urge reflex contraction (urge to void) is about
400mL.
Sympathetic nerve to bladder play no part in
micturition, but they mediate the contraction of the
internal urethral sphincter that prevent semen
from entering the bladder during ejaculation.
Reflex and Voluntary Control of Micturition
Reflex Control
Voluntary Control
Bladder fills
Cerebral cortex
+
Stretch receptors
+
+
Motor neuron to
external sphincter
Parasympathetic nerve
+
Bladder
Bladder contracts
External urethral
sphincter opens when
motor neuron is inhibited
Internal urethral sphincter
mechanically opens when
bladder contracts
Urination
External urethral
sphincter remains
closed when motor
neuron is stimulated
No urination
Cystometry
Study the relationship between intravesical
volume and pressure.
Done by inserting catheter and emptying the
bladder, then recording the pressure while
bladder filled at 50ml increment of water.
This plot is known as the cystometrogram.
Cystometrogram
Filling or Storage Phase
Bladder Pressure (mmHg)
60
Micturition
Reflex
50
II
(400mL)
40
Voiding
30
20
Ib
(150mL)
10
Ia
0
0
mV
100
200
300
400
500
Bladder Volume (mL)
EMG of External Sphincter
600
Plot has 3 components (segments):
Ia – initial slight rise in pressure when the first
increment in volume are produced
Ib – a long, nearly flat segment as further
increments are produced (conscious level to
void at about 150mL)
II – a sudden, sharp rise in pressure as the
micturition reflex is triggered (sense of
fullness and urge to void at about 400mL)
Laplace Law
The flatness of segment Ib is a manifestation
of the law of Laplace, which states that the
pressure in the spherical viscus equal to
twice the wall tension divided the radius.
P = 2T / r
THUS, MICTURITION REFLEX IS A SINGLE
COMPLETE CYCLE OF:
1) Progressive and rapid increase of pressure,
2) A period of sustained pressure, and
3) Return of the pressure to the basal tone of
the bladder
Storage
Bladder is a reservoir that stores urine (400-600 ml,
Usually fills at a constant rate (slower at night,
faster with bladder irritants).
Accommodation – detrusor is elastic and allows
filling by staying relaxed thus maintaining low
bladder pressure.
The trigone stays closed during filling.
Storage
•
•
Postponement of voiding
– Signals go to sacral micturition center
– Travels up to the pons
– Via lateral spinothalamic tract
– Result - brain inhibits sacral center, bladder
keeps filling.
Outlet pressure is higher than bladder pressure
Emptying
•
At critical level of filling
– Stretch receptors produce a strong
sensation to void – urge
–
Urgency – is sudden compelling desire to
void which is difficult to stop.
–
Signals go from bladder to brain (pons)
–
–
Signal returns via somatic track
Decision is made to get to the toilet
Autonomic Control of Micturition
Type of nerve
Name of nerve
Spinal
innervation
Action
Somatic
Pudendal nerves
Nerve to the
levator ani
S2-4
Sensory and
voluntary motor to
external sphincter
Sympathetic
Hypogastric
nerves
T11-L2
Detrusor relaxation
Internal sphincter
contraction
Parasympathetic
Pelvic nerves
S2-4
Inhibit sympathetics
causing detrusor
contraction
Internal sphincter
relaxation
Changes with aging include:
•
Decline in the number of functional nephrons
• Reduction of GFR
• Reduced sensitivity to ADH
• Problems with the micturition reflex